Chairs in which the seat and back tilt as a unit usually include a base providing a fulcrum for the seat-back unit, and some sort of spring mechanism tending to bring the chair into erect position. This mechanism opposes the tilt with a progressively increasing force as the tilt angle increases. The fulcrum axis is normally underneath the seat, which brings the center of gravity of the occupant progressively to the rear as the tilt angle develops. This type of mechanism is frequently found in office swivel chairs, but is also appearing with greater frequency in residence furniture.
Spring-action chair mechanisms tend to become noisy with wear, or to become quite costly if sufficiently well built to reduce this tendency. The characteristic increasing resistance to tilt also requires the occupant of the chair to continually provide some degree of force against the floor, or against some other object, in order to maintain a tilted position. Over long periods of time, the resulting muscle strain becomes particularly noticeable. On release of this holding force, the occupant is immediately thrown forward as a result of the spring action.
A tilting mechanism that does not use a return spring is shown in my U.S. Pat. No. 3,227,491. A frictional resistance is substituted for the spring action, and the degree of resistance is preferably controlled so that it decreases with an increased angle of tilt. In the preferred form of the invention disclosed in that patent, this resistance gradient is accomplished by the use of an arcuate braking surface which is slightly eccentric to the axis of the fulcrum. A spring driving a plunger against the braking surface is progressively compressed as the chair approaches the erect position, and is extended as the chair tilts backward. The degree of resistance is sufficient to maintain the position of the occupant under normal conditions, and yet is free enough so that he can move back to erect position by slightly shifting his position. The tilt position is therefore a function of resistance and balance.
A chair of the type shown in U.S. Pat. No. 3,227,491, has an objectionable tendency to assume an angle of tilt when unoccupied. This is due primarily to the absence of any automatic return system, such as is present in the usual spring machanism. An accidental brushing against the chair, or an attempt to pull the chair across the floor by grasping the back, will inevitably produce a degree of tilt and affect the appearance of the chair. The type of mechanism shown in that patent has also been somewhat difficult to incorporate in certain designs of furniture without presenting some degree of distortion of the lines of design.